feat: add PDF support via mupdf WASM

[ndemarchis]

Summary

• Adds PDF → text/markdown conversion using the mupdf npm package (AGPL-3.0, same license as VERT), running entirely client-side in a web worker
• Adds generic converter chaining so formats can be converted through intermediate steps (e.g. PDF → md → docx via mupdf + pandoc)
• The mupdf WASM binary is not committed — a postinstall script copies it from node_modules

Supported conversions

• PDF → md, txt (direct via mupdf)
• PDF → docx, html, rtf, rst, epub, odt, docbook, json (chained through pandoc)

[JovannMC]

note: this is a boilerplate response for numerous open PRs

hiya!

firstly sorry for the late response to your PR (if it has been open for a while). we are closing this, along with other people's PRs, as we are preparing for the very overdue update drop. once merged, it will most likely break most existing PRs and will need to be redone. track the PR #282, and once merged you may wish to open another PR with the new base.

do also note this repository's and our main contributing guidelines which was made recently here, when submitting your new PR: repo & main